In a radio communications system in which OFDM (Orthogonal Frequency Division Multiplexing) technology, for example, is used in a digital modulation scheme, a transmission channel is estimated using pilot signals (pilots). The pilots are known to both transmitting and receiving ends.
FIG. 1 shows an example of an arrangement of pilots. In FIG. 1, pilots are mapped in resource elements indicated by hatching. In resource elements other than the hatched ones, data are mapped. A resource element here is defined as a radio resource indicated by one subcarrier and one symbol.
A mobile station apparatus makes transmission channel estimates of resource elements in which pilots are mapped, using an equalization scheme. Examples of the equalization scheme are Zero Forcing Equalization and MMSE (Minimum Mean Square Error). Also, the mobile station apparatus makes transmission channel estimates of resource elements in which no pilots are mapped, based on the result of the estimates of the resource elements with pilots mapped. To make estimates of the resource elements with no pilots mapped, the mobile station apparatus carries out, for example, interpolation in the time (i.e. symbol) direction and the frequency (i.e. subcarrier) direction using the result of the estimates of the resource elements with pilots mapped.
Next is described the case of performing transmission channel estimation by interpolation.
When a sampling function is used, an allowable delay spread Td is determined by a pilot spacing Nf (Nf is an integer larger than 0) in the frequency direction. Also, an allowable Doppler frequency fd is determined by a pilot spacing Nt in the time direction. The term “allowable” here refers to being able to perform interpolation. The relationships between the allowable delay spread Td and the allowable Doppler frequency fd are expressed by the following relational expressions (1) and (2).Td<1/(Nf·f0)  (1)fd<1/(2Nt·Ts)  (2)
In the relational expressions (1) and (2) f0 is a subcarrier frequency spacing, and Ts is a symbol length of OFDM.
According to the relational expressions (1) and (2), if the delay spread Td is large or the Doppler frequency fd is high, the relational expressions (1) and (2) can be satisfied by arranging pilots close to one another to be thus densely spaced. For example, as illustrated in FIG. 2, the pilot spacings in both the frequency direction and the time direction are made smaller compared to the case of FIG. 1.
The pilot spacings can be set larger, for example, if transmission channel characteristics are favorable compared to the case where they are poor. The pilot spacings may be dynamically changed between the mobile station apparatus and a base station apparatus in communication with the mobile station apparatus.
Such a mobile station apparatus is described below with reference to FIG. 3.
Downlink signals are transmitted from a base station apparatus. In the downlink signals, pilots are mapped, as illustrated in FIG. 1. In the example of FIG. 1, pilots are spaced every 6 resource elements apart in the frequency direction and every 4 resource elements apart in the time direction.
In a mobile station apparatus 1, a downlink signal is received by a radio reception unit 2. The received downlink signal is input to an FFT (fast Fourier transform) unit 4. The FFT unit 4 performs a fast Fourier transformation on the input downlink signal. The downlink signal after the fast Fourier transformation is input to a speed measurement unit 12, a transmission channel estimation unit 6 and a compensation unit 8. The transmission channel estimation unit 6 performs transmission channel estimation using pilots included in the fast Fourier transformed downlink signal, and then inputs the estimation result to the compensation unit 8. The compensation unit 8 makes compensation to the fast Fourier transformed downlink signal based on the estimation result input from the transmission channel estimation unit 6.
The arrangement of the pilots included in the downlink signal is already known to the mobile station apparatus 1. For example, it may be arranged at the time when the base station apparatus and the mobile station apparatus 1 initiate communications such that the arrangement of pilots included in a first downlink signal transmitted from the base station apparatus is the same as the arrangement of pilots included in a first downlink signal received by the mobile station apparatus 1. As a result, when communications are established between the base station apparatus and the mobile station apparatus 1, the arrangement of the pilots included in the downlink signal is known to both the base station apparatus and the mobile station apparatus 1. The downlink signal compensated based on the result of the transmission channel estimation is input to a decode unit 10. The decode unit 10 decodes the input downlink signal. The decoded data are used as received data.
On the other hand, the speed measurement unit 12 measures the speed of the mobile station apparatus 1 based on the fast Fourier transformed downlink signal. The measured speed information is input to a pilot arrangement determination unit 14. The pilot arrangement determination unit 14 makes a determination on the arrangement of pilots based on the input speed information. For example, if the speed is fast, the pilot arrangement determination unit 14 may determine the necessity to space the pilots more densely. As has been explained with reference to FIG. 2, the pilot arrangement determination unit 14 may determine to, for example, space the pilots every 3 resource elements apart in the frequency direction and every 2 resource elements apart in the time direction. The pilot arrangement determination unit 14 inputs the determined pilot arrangement to the transmission channel estimation unit 6 and the compensation unit 8. The pilot arrangement determination unit 14 also inputs information indicating the determined pilot arrangement to a control channel encode unit 16. The control channel encode unit 16 encodes the input information as control information, and inputs the control information to a multiplexing unit 18. The multiplexing unit 18 multiplexes, with an uplink data channel, the input control information as an uplink control channel. The multiplexing unit 18 then inputs, to a modulation unit 20, an uplink signal into which the uplink control channel and the uplink data channel are multiplexed. The modulation unit 20 performs a modulation process on the uplink signal. The modulation unit 20 inputs the modulated uplink signal to a radio transmission unit 22. The radio transmission unit 22 transmits the input uplink signal to the base station apparatus.
The base station apparatus arranges pilots in a downlink signal in accordance with the pilot arrangement carried by the uplink control channel. Then, the base station apparatus transmits the downlink signal.
The mobile station apparatus 1 receives the downlink signal at the radio reception unit 2. Then, the FFT unit 4 performs a fast Fourier transformation on the downlink signal. The transmission channel estimation unit 6 performs transmission channel estimation based on the pilot arrangement input from the pilot arrangement determination unit 14. The compensation unit 8 makes compensation to the fast Fourier transformed downlink signal based on the estimation result obtained by the transmission channel estimation unit 6. The compensated downlink signal is input to the decode unit 10, which then decodes the input downlink signal. The decoded data are used as received data.
The transmission channel estimation unit 6 and the compensation unit 8 (part indicated by a dashed line in FIG. 3) are described below with reference to FIG. 4.
Assume that an FFT output of an OFDM receiver is obtained by the following expression (3).X(k,l)=H(k,l)d(k,l)+Z(k,l)  (3)
H(k,l) is a transmission channel characteristic corresponding to a first carrier wave (subcarrier) of an OFDM symbol at a time k. Assume that d(k,l), which is a transmission symbol, is known to the OFDM receiver. Z(k,l) is an additive noise. A transmission channel estimate H′ (k,l) is obtained by the following expression (4).
                                                                                          H                  ′                                ⁡                                  (                                      k                    ,                    l                                    )                                            =                                                X                  ⁡                                      (                                          k                      ,                      l                                        )                                                  /                                  d                  ⁡                                      (                                          k                      ,                      l                                        )                                                                                                                          =                                                H                  ⁡                                      (                                          k                      ,                      l                                        )                                                  +                                                      Z                    ⁡                                          (                                              k                        ,                        l                                            )                                                        /                                      d                    ⁡                                          (                                              K                        ,                        l                                            )                                                                                                                              (        4        )            
d(k,l) is an example of pilots generated by a pilot generation unit 24 in FIG. 4. A complex division unit 28 calculates the expression (4).
The pilot generation unit 24 generates pilots, and inputs the generated pilots to the complex division unit 28. A pilot pattern selection unit 26 selects pilots from fast Fourier transformed symbols input from the FFT unit 4, and inputs the pilots to the complex division unit 28. The complex division unit 28 obtains transmission channel characteristics corresponding to the pilots input from the pilot pattern selection unit 26 based on the pilots input from the pilot generation unit 24, and inputs the obtained transmission channel characteristics of the pilots to a time-direction interpolation unit 30. The time-direction interpolation unit 30 performs interpolation in the time direction. The time-direction interpolation unit 30 inputs, to a frequency-direction interpolation unit 32, transmission channel characteristics corresponding to the pilots based on which the time-direction interpolation has been performed. The frequency-direction interpolation unit 32 performs interpolation in the frequency direction. The frequency-direction interpolation unit 32 inputs, to a complex division unit 34, transmission channel estimation results obtained after the frequency-direction interpolation. Based on the transmission channel estimation results, the complex division unit 34 estimates transmitted symbols, which are then input to the decode unit 10.
A scheme referred to as the scattered pilot (SP) scheme is one way to insert pilots. One example of a scattered pilot arrangement is explained with reference to FIG. 5.
In the case where the scattered pilot scheme is applied, the tolerance of the delay spread can be increased by, for example, performing the time-direction interpolation first, and then performing the frequency-direction interpolation using the result of the time-direction interpolation. In the case of FIG. 5, if the time-direction interpolation is performed first, the resource element spacing Nf, with which transmission channel characteristics in the frequency direction are obtained, can be changed from 9 to 3, and therefore, the tolerance of the delay spread becomes three times larger.
Japanese Laid-open Patent Application Publication No. 2007-150971
In the case where pilots are spaced densely, as described above, resource elements allocated to data decrease, which in turn results in a decrease in the throughput. Given this factor, the scattered pilot scheme is applied to thereby increase the tolerance of the delay spread without increasing the number of pilots.
In this case, however, although the tolerance of the delay spread can be increased, the tolerance of the Doppler frequency is reduced.